1. Field of the Invention
Embodiments of the present invention relate to a composition for carboxylic acid production and methods for making and using same.
More particularly, embodiments of the present invention relate to a composition for carboxylic acid production and methods for making and using same, where the composition includes a metal catalyst, carbon monoxide, an alkanol, an iodine source, an additive providing hydrogen bonding to HI present in the reaction media, and optionally added water.
2. Description of the Related Art
Production of acetic acid via methanol carbonylation is an industrial process used on a global basis to produce billions of pounds of glacial acetic acid. The process as currently practiced commercially operates under relatively mild conditions with high selectivity for utilization of methanol and carbon monoxide raw materials.
A large body of literature pertaining to this process exists and a review of this literature indicates several key operating parameters that must be considered for efficient process operation. One of these parameters concerns maintaining a low concentration of water in the reactor. This requirement is obvious; glacial acetic acid is marketed with a water concentration in the parts per million range. Thus, reactor water above ppm levels must be removed, requiring energy—the more water, the higher the energy requirement.
Those skilled in the art are familiar with the original literature by Monsanto (see, e.g., U.S. Pat. No. 3,769,329, inventor of the basic modern process) and a reading of this literature indicates a second parameter concerning catalyst stability. Catalyst stability is adversely affected by lower reactor water concentrations. Moreover, increasing water increases by-product formation due to such reactions as the water gas shift reaction, which is directly proportional to the reactor water concentration.
Another problem with the current processes is that catalyst stability and activity is lower than desired even with high reactor water concentrations. The literature indicates that oxidative addition of hydrogen iodide to the active rhodium species is the first step in a series of reactions leading to both the undesired water gas shift reaction and to rhodium precipitation. See, e.g., N. Hallinan and J. Hinnenkamp, Rhodium Catalyzed Methanol Carbonylation: New Low Water Technology, Proceedings of the Organic Reactions Catalysis Society, 2000. The reference disclosed one can inhibit the addition of hydrogen iodide to the rhodium species through the interaction of HI with a weak base such as a tertiary phosphine oxide. The reference further disclosed that if too strong a base is used, the hydrogen iodide intermediate is removed from the iodide cycle such that methyl iodide is not regenerated from methyl acetate and the reaction rate drops significantly and even stops. Thus, selection of an additive with a base strength within a very narrow range is effective in maintaining the rhodium in the desired active and stable form independent of reactor water concentration.
Thus, there is a need in the art for an improved carboxylic acid preparation process. The new processes and catalyst compositions provide a means to provide improved catalyst stability, reduced energy consumption, and reduced by-product formation. Because the catalyst metals used in this process are expensive (e.g., Rh, Ir, Pd, etc.), improving catalyst stability and simplifying acetic acid recovery can decrease metal loss and reduce catalyst regeneration.